1. Field of the Invention
This invention relates to a cobalt(III) complex-containing photographic composition, element and process which can be made negative-working or positive-working, depending upon the nature of the exposure given.
2. State of the Prior Art
Cobalt(III) complexes containing releasable ligands have been utilized in non-silver redox reactions photoinitiated by means of photoreductants or spectral sensitizers, as disclosed, for example, in Reseach Disclosure, Volumes 126 and 130 October 1974 and February 1975, Publication Nos. 12617 and 13023, respectively, published by Industrial Opportunities Limited, Homewell, Havant Hampshire PO91EF, United Kingdom. Preferred reduction products of such a reaction are cobalt(II) and amines such as ammonia. Either of these can in turn be used to form images, the cobalt(II) being chelated by compounds capable of forming at least bidentate chelates, or the amines being reacted with dye precursors, including diazo-coupler systems, to form a dye; or the cobalt(II) or amines can be used to bleach out preincorporated dye. Such imaging processes are disclosed in Research Disclosure, Volume 126, October 1974, Publication No. 12617, part III. Amplification can be achieved by using a reducing agent precursor capable of forming a reducing agent with the aforesaid reduction products, for the reduction of remaining cobalt(III) complexes. For example, certain of said chelating compounds for cobalt(II) form, when chelated, a reducing agent, as described in Research Disclosure, Volume 135, July 1975, Publication No. 13505, and o-phthalaldehyde will react with ammonia to form a reducing agent, as disclosed in Research Disclosure, Volume 158, June 1977, Publication No. 15874. Imaging elements based upon such reactions involving cobalt(III) complexes are useful, for example, in graphic arts as room-light handling contact films, e.g., as an intermediate in the preparation of printing plates.
The cobalt complex imaging elements of the prior Research Disclosure can be either negative-working or positive-working, but the same element is not both. This is, a separate image-forming material (e.g., a dye former) is used in those instances in which the element is to be negative-working, compared to those instances in which the element is positive-working (e.g., by bleaching out a pre-existing dye). It would be highly desirable to provide an imaging element wherein the same element can be made negative or positive working, merely by controlling the processing of the element.
A further aspect of said prior cobalt complex imaging elements is that they can develop background print-up because the unexposed photoinitiator in the background areas causes increased D.sub.min (minimum density) upon handling as a result of room light activation of the photoinitiator. To avoid print-up, a peel-apart system can be used, that is, one wherein the layer of cobalt(III) complex relied upon for the imaging is removed from the image recording layer. Print-up is more difficult to avoid in integral systems, particularly when the exposed photoreductant reduces the complex at room temperatures.
Therefore, there is a need for cobalt imaging systems which are less likely to suffer print-up when handled in room light conditions.
Metal complexes capable of releasing amines have been used prior to this invention to thermally activate image recording means such as diazo-coupler compositions. Examples are disclosed in U.S. Pat. Nos. 3,469,984, issued Sept. 30, 1969; 3,224,878, issued Dec. 21, 1965; and 2,774,669, issued Dec. 18, 1956. Because of their thermal instability, such complexes generate amines when overall heated, to cause diazo coupling or the like. There is no disclosure in this art of means for inhibiting such dye development.
Canadian J. Chem., Volume 54, pages 3685 through 3692 (1976), teaches that certain cobalt complexes, specifically [(NH.sub.3).sub.5 Co(III)dimethylsulfoxide].sup.3+, can be inhibited against thermal reduction by the addition of equimolar concentrations of p-toluenesulfonic acid. No suggestion is made that such acid be photolytically generated.
Patents relating to the background of image formation using amines or cobalt complexes include U.S. Pat. Nos. 2,774,669, issued Dec. 18, 1956; 3,102,811, issued Sept. 3, 1963; 3,469,984, issued Sept. 30, 1969; 3,224,878, issued Dec. 21, 1965; and Japanese Patent Publication No. 74/6234.